The present invention relates to a new silica powder composition in which the powder particles comprise hollow shells of amorphous hydroxylated silica, and, more particularly, to a process for preparing such hollow silica shells in which their size and shape can be precisely controlled. The ability to control the size and shape of the silica shells renders them suitable for a number of uses, such as, for example, fillers in plastics and elastomers, as extenders in paint and paper, and as anti-caking agents in commercial powder products.
Porous hollow silica shells have been used for micro-encapsulation of insecticides as described in Nakahara et al. in Zariyo Gijutsu, Vol. 5, No. 5 (1987), pp. 231-236. The shells consist of individual silica particles which have been bonded together to form a porous assemblage. The shells are prepared by aggregating silica particles on the surface of droplets in a water/oil emulsion and recovering the product by centrifuging and filtering. The shells are then washed, dried and sintered at a temperature of 500.degree. C. for 10 hours.
Darley et al., Planta, 130, (2), (1976), pp. 159-167 and Sadava et al., Planta 135, (1), (1977), pp. 7-11, describes the formation of silica shells by sequential deposition of silica within a membrane structure.
U.S. Pat. Nos. 2,885,366 and 2,913,419 relate to coating a solid core material with a coating layer or skin of silica.